Screw extruder with radially projecting pins

ABSTRACT

A screw extruder for the continuous processing of a synthetic thermoplastic material in which one section of the rotatable screw in the metering or discharge zone of a barrel extruder carries a plurality of radially projecting pins of different specific heights in place of the thread flight, this section extending along an axial length of at least two screw diameters. This combination of apparatus is particularly useful in evening or equalizing the temperature of the processed thermoplastic material.

United States Patent [1 1 Hensen et a1.

1 1 3,751,015 1 Aug. 7, 1973 1 SCREW EXTRUDER WITH RADIALLY PROJECTINGPINS [75] Inventors: Friedhelm Hensen, Remscheid;Hans Siemetzki, Hilgen;Egon Gathmann, Huckeswagen, all of Germany [73] Assignee: Barmag BarmerMaschinenfabrik 1 Aktiengesellschaft, Wuppertal,

Germany [22] Filed: June 22, 1971 {21] Appl. No.: 155,458

[52] US. Cl 259/191, 100/145, 425/205 [51] Int. Cl. B291 3/01, B29b 1/06[581 Field ofSearch ..259/8-10, 191-193,

{56] References Cited UNITED STATES PATENTS 2,838,794 6/1958 Munger cta1. .1 259/191 3,160,688. 12/1964 Aykanian 264/53 5/1966 Colombo 259/1923,487,503 1/1970 Barr et a1. 425/208 3,591,146 7/1971 Sutter e 259/103,595,533 7/1971 Sutter 425/205 3,623,254 1/1972 Woodham....... 425/2053,652,064 3/1972 Lehnen et a1... 425/208 3,680,844 8/1972 Menges eta1... 425/205 Primary Examiner-William T. Price Assistant ExaminerStuartS. Levy Attorney-Johnst0n, Root, OKeeffe [57] ABSTRACT A screw extruderfor the continuous processing ofa synthetic thermoplastic material inwhich one section of the rotatable screw in the metering or dischargezone of a barrel extruder carries a plurality of radially projectingpins of different specific heights in placeof the thread flight, thissection extending along an axial length of at least two screw diameters.This combination of apparatus is particularly useful in evening orequalizing the temperature of the processed thermoplastic material.

10 Claims, 5 Drawing Figures PATENIED SHEEI 1 0F 2 INVENTORS:

FRIEDHELM HENSEN HANS SIEMETSKI ATT'YS PAIENTEDMJB m SHEET 2 0F 2 0 00 d0 00 w mvawyw 000 000 0 Q 000 000 000 000 000 000 OOOOOOWOOOOGOOOOO 0NWO 00000000000000000 OOOOOOOOOOOOOOOOO 000 00 0 w 0 00 OO O@O O mO m000 m 00 0 go 000 000 m 0 000 000 000 000 IENVENTORS:

FRIEDHELM HENSEN HANS SIEMETSKI EGON GATHMANN y fl z ATT'YS SCREWEXTRUDER WITH RADIALLY PROJECTING PINS This invention generally relatesto screw extruders adapted to continuously process a syntheticthermoplastic polymer material by means of a rotatable single worm orscrew which is provided on a core or root surface with radiallyprojecting pins spaced from one another. These pins or pegs are arrangedon the periphery of the screw core and provide a device for the thoroughmixing and homogenizing of the more or less uniform thermoplasticpolymer melt which is present in the metering or pumping zone of theextrusion'press where this melt essentially flows in laminar andnon-turbulent streams.

Screws provided with various types of projections or stubs'are known;Depending on the intended purposes, e.g., a thorough mixing alone or athorough mixing in conjunction with a homogenizing of the molten plasticmaterial, the projections are provided either in the feed zone or alsoin the so-called compression or transition zone or sometimesin themetering or discharge zone of the screw extruder. Screws with suchprojections on the root or core have been proposed in various differentembodiments, and these differ substantially according to the type ofconstruction, placement and/or arrangement of the projections.

ln a kneading press for alimentary pastes (pastry dough) according toU.S. Pat. No. 2,620,752, which consists essentially of a double worm anda single worm arranged transversely to it, there is disclosed a numberof individual radial projections or knobs arranged along the spiralchannel on the periphery of the screw core between adjacent threadflights. In providing these projections between the thread flights ofthe usual screw channel, they form over a length of several screwchannels a single row of cap-shaped or hemispherical kneading means, allof which are constructed alike according to shape, size and height.

Furthermore, it has been suggested in U.S. Pat. No. 2,838,794 to provideshallow projections on a screw in an extrusion apparatus for processingthermoplastic material, which projectionsact as a resistance to the meltflow and are described as helping to maintain proper frictional forcesbetween the screw and the plastic. The screw according to this patenthas for this pur pose numerous small, sharp-edged projections of acertain geometric shape and size as well as a specific very low height.All of these projections are constructed so as to be equal in shape,size and radial height. The projections are arranged parallel to eachother in rows between the conventional thread flights of the screwchannel in the feed zone of the screw extruder.

For the thorough mixing of a thermoplastic polymer melt in a screwextruder. it has further been proposed to arrange elongated projectionsin the screw channel between the thread flights. These elongatedprojections alsoextend radially from the outer surface of the core orroot of the screw toward the screw casing or housing, their outersurface being aligned with the outer surface of the adjacent threadflights, i.e., the surface of the flight lands. At least one row ofthese projections is provided along at least one side of the thread, andthis is disposed transversely in the screw channel. There can also bearranged several rows of such elongated projeetions adjacent andparallel to each other, and they can be arranged both in the transitionzone and also in the metering zone of the screw extruder. in all cases,however, such projections are situated only between adjacent threadflights of the normal screw channel. These projections essentially formthese cams partial streams of the plastic in the screw channel. In thecase of relatively high discharge velocities, for example with a screwturning rate in the region of revolutions per minute and with use of ascrew having relatively large thread depth, e.g., in the area of 5 mm.depth, as well as with individual projections whose free radial heightprojecting outwardly beyond the screw core or root always corresponds tothe size of the screw thread depth and corresponding to a diameter abouton the same order of magnitude, it is possible in the processing ofhighly viscous thermoplastic materials to achieve a reasonableuniformity with respect to temperature, mixing and, where required, alsowith respect to dyeing or pigment coloration of the thermoplasticpolymer. The thorough mixing achieved is generally satisfactory for awide variety of thermoplastic materials subjected to extrusion molding.The temperature equalization between the hotter core melt and the outerperipheral melt generally is sufficient to provide a temperaturedifferential (AT) which lies between about 3 and 5 C.

In all the above mentioned proposals, the flowing thermoplasticmaterial, when in the form of a melt, is more or less intensively mixedby the projections provided within the continuous spiral screw channellaterally bounded by the thread flights. However, just as before theintroduction of these projections, the screw channel determines the maindirection of flow. In this manner the intended mixing effect isconsiderably influenced. On the other hand, it is not possible toachieve in this manner for all thermoplastic materials a favorablemixing effect in combination with a favorable temperature distributionor equalization.

A favorable result with respect to thorough mixing and homogenizationofa thermoplastic material melt is achieved by wart-like studs orprojections in the area immediately before the discharge region of ascrew extruder. Thus, in the extruder according to German publishedapplication No. 1,289,302, there are provided for this purpose suchwart-like projections situated alternately on the end surface or taperedhead of the screw as well as on the opposing surface of the barrelhousing which may be a correspondingly tapered mouth or throat of anozzle. These projections, preferably in the form of spirally orientedbodies, are arranged in a series of successive coaxial circlesconcentric to the screw axis.

The melt flowing laminarly in the discharge zone by reason of its highviscosity is subdivided in a positive or forced manner by the oppositelysituated projections, which can be compared with bosses or baffles oflow height, in which process the constituent portions of the melt areset into turbulence and thus mixed with one another. By reason of thisthorough mixing effect, there is likewise an active influence on thetemperature variation in the melt. The turbulence of the meltconstituents brought about by the projections moved past one another incomb-like manner, however, depending on the type of thermoplasticmaterial to be processed in the screw extruder, can lead to the resultthat additional frictional heat is developed which is locally very high.There is obtained, to be sure, a largely homogeneous and well mixed meltby reason of using the described projections, but a favorabletemperature distribution is not to be achieved since too little timeremains for equalization of the melt temperature with this action of themelt.

Such a solution is disadvantageous in a screw extruder insofar as thenumber of projections is necessarily restricted because of theiralternate arrangement on the end surface of the screw and the interiorsurface of the housing lying opposite it. If, nevertheless, a largenumber of projections should be provided in such a structure, then thescrewheads or projections are to be arranged in the radial direction onthe cylinder part of the screw end and on the corresponding part of theopposing nozzle or housing wall. In this case there are yieldeddisadvantages with respect to manufacturing and installation of variousparts since the cylinder or barrel housing wall must now be providedwith additional cavities and threads for the projections, and theinstallation of the worm or screw of the extruder is rendered much moredifficult.

One object of this invention is to provide an extruder in which asynthetic polymer melt is to be subdivided in the metering zones of thescrew with the aid ofa mixing device or element which is located only onthe screw and which is simple in construction but extremely efficient.This mixing device or element should break up the polymer melt into alarge number of individual melt streams which are then differentlyunited, i.e., recombined in a different order in sequence and position.

Another object of the invention is to provide an improved screw extruderwhereby the melt is presented at the discharge opening of the screw withexcellent temperature equalization as well as being optimally intermixedand homogenized.

It is a further object of the invention to provide screw extruderapparatus which is equally effective with low, medium and high workingrates of the screw and which works equally well in screws with a normalL/D ratio as well as an unusually large or small L/D ratio. Moreover,the apparatus is to be equally applicable for processing highly viscousas well as more fluid thermoplastic materials.

These and other objects and advantages of the invention will become moreapparent upon consideration of the following detailed specification,

It has now been found, according to the present invention, that suchobjects are effectively achieved in an otherwise conventional screwextruder for continuously processing a thermoplastic polymer, byproviding at least one section of the screw in the metering or pumpingzone of the extruder which is free of thread flights over an axiallength of at least two times the screw diameter and which carries alarge number of pins projecting radially from the threadless screw core,the individual pins being varied between at least two different freeradial heights and the ratio of pin diameter to said free height beingfrom 0.25:1 to 2:l. Pins with different heights are preferably presentas long and short pins. The threadless screw section must have a lengthof at least about 2D (two screw diameters), the upper limit being lesscritical and selected for reasons of economy, e.g., up to about 4D. Asthe threadless section of the screw core, there can be provided the freescrew end or one or more arbitrary sections of the screw in the meteringzone or both of these in common, i.e., from the beginning of themetering zone up to the screw end at or adjacent to the discharge outletof the extruder. The metering zones of extruders represent a definitelength of the screw which usually exhibits a different channelcross-section or different thread pitch than the feed or transitionzones.

The ratio of the pin diameter to its free radial height is desirablymaintained within the above noted limits of 0.25 to 2.0. The short andlong pins provided in large number are alternately and/or irregularlyarranged along the threadless core or root of the screw section.

The exact number of pins or projections required on the threadless screwsection is somewhat difficult to define in view of their variation insize, shape, position and the like. In general, however, approximatelycylindrical pins are especially useful which are located in closelyspaced relationship on at least half and preferably two-thirds or moreof the 2D peripheral length of the threadless core or root of the screw.In addition, it is estimated that the total surface area of all of theradially projecting pins, measured as the total exposed surface area ofcylinders which project radially outwardly from the root of the screw,should preferably amount to at least about one-fourth and preferablyone/third or more additional surface area than that which wouldotherwise be provided by the threadless core or root alone, i.e., freeof threads and projections. This means that there must be a verysubstantial number of pins with a relatively large radial height.

It is also very desirable to provide a clear spacing or flow gap of notmore than l.5 times a pin diameter between adjacent cylindrical pins, apreferred embodiment of the invention employing pins which are all equalor approximately equal in diameter. Since the channel depth or spacingbetween the root surface and wall of the barrel is relatively small inthe metering zone, the limited ratio of height to diameter of theindividual pins will generally require at least about 10 pins on anaxial line over the threadless section of the screw, depending upon theexact length of this section which -is at least two times the screwdiameter.

A number of embodiments of the invention together with certain preferredfeatures are further described in conjunctionwith the accompanyingdrawing in which:

FIG. 1 is a partial side view of one cutaway segment of the screw in ametering zone with the inner walls of the barrel housing indicated incross-section, this particular screw carrying radially projecting pinsof different diameters and varying free radial heights;

FIG. 2 is a partly schematic peripheral layout of the pin carryingthreadless section of the screw where the pins have been arranged in aso-called fishbone or herringbone pattern over this length of the screw;

FIG. 3 is a cross-sectional view taken on line 3-3 of FIG. 2, i.e.through the cylindrical root or core of the screw with the pins placedas in FIG. 2;

FIG. 4 is a cross-sectional view through another embodiment of the screwto illustrate the location of the pins in the metering section adjacenta final screw thread at the free end; and

FIG. 5 is a cross-sectional view of still another screw embodiment toillustrate the location of pins on the free end of the screw after thefinal screw thread of the metering section.

Referring first to FIG. 1, the screw 1 is rotatably driven in the barrelhousing 2 of a screw extruder for continuously processing thermoplasticmaterials, with a root or core 3 of the screw bearing thread flight 4which terminates just before the mixing device or special screw section3 of the invention. The mixing device essentially consists of a largenumber of radially projecting pins of differing height, in which thelong pins are generally designated as 5 and the short pins as 6. Theindividual pins are preferably shrunk and press fitted intocorresponding radial bores in a section of the screw free of the threadflight, for example, that por tion of the core 3' of an arbitrarilyselected screw section within the conventionally required metering zoneor zones. The metering zones are essentially all of the extruder zoneswhich occur after the compression or transition zone in the extrusiondirection. These screw zones differ, for example, in having differentcore cross'sections, the metering zone generally having the greatestcircular cross-section and the smallest channel height or thread depth.See, for example, the basic extrusion fundamentals disclosed in Chaptereleven of Plastics Extrusion Technology by Griff, Van Nostrand ReinholdCo., 2nd Edition (1968).

In the embodiment according to FIG. 1 there are irregularly andalternately arranged long and short pins 5 and 6, respectively, whichare interspersed with other smaller diameter pins 6a. The ratio of pindiameter to free radial length lies in the range between 0.25 and 2.0.The clear spacing or flow gap from pin to pin is chosen irregularly,however, with reference to the larger diameter pins 5 and 6, this gap issmaller than 1.5 pin diameters. On the basis of this design of the pinlocations and of the arbitrary arrangement of the individual pins, thereis obtained at least one threadless screw section in the metering zoneor zones of the extruder wherein such pins provide free flow gaps forthe thermoplastic melt, the gaps being of differing heights and widths.

In the embodiment according to FIGS. 2 and 3, the pins of equal diameterare arranged in circumferentially and/or axially spaced groups,.partlyin axially parallel rows 7 and partly in rows 8 at an angle to the screwaxis. Such an arrangement is called a fishbone" or herringbone pattern.Preferably, every three axial pin rows 7, for example an inner row withshort pins 6 and two outer rows with long pins 5, form one pin group Aand every three of the transversely angled pin rows 8, for example amiddle row with short pins 6 andtwo outer rows with long cams 5, form apin group B. The pin rows 8 thus intersect the pin rows 7 at an angle.The direction of extrusion is indicated by the arrow on the right-handside of the peripheral layout. The pin groups B are arranged at specificbut selectible lateral or axially directed spacings relative to oneanother and obliquely to the screw axis. Adjacent pin groups B form,with the aid of the two pin groups A, a number of separated screw zonesC which are free of pins, threads or any other projections.

It is possible for the arrangement of the pins to be such that the zonein which the pins are located extends into the screw channel bounded bythread flights and adjacent to a threadless screw section, as indicatedfor example in FIG. 4.

Moreover, it is possible and often preferred to provide the specialmixingsegment of the invention along the Iastportion of the meteringzone as shown in FIGS. 4 and 5, either before at least one final flightland 9 as in FIG. 4, or on the screw head itself 10 extended over alength of at least twice the screw diameter as indicated in FIG. 5following termination of the metering screw flight 11. Again, the arrowin these embodiments indicates the direction of extrusion produced bythe screw, the previous feed and transition zones being omitted sincethey are constructed in conventional manner.

The discharge outlet or ejection portion of the screw extruder whichfollows the metering zone, e.g., along or just beyond the free end ofthe screw, is also capable of being quite conventional in its design asa smooth bored segment of the barrel housing which does not require anyspecial profiling or insertion of projections or other baffles incoaction with the radial pin projections of the invention. Also, thisdischarge outlet can be equipped with conventional screens and/orbreaker plates as well as valve means to control the flow and pressureof the melt. Known heating and/or cooling means can also be applied tothe screw extruder, e.g., the usual electrical heaters operated asresistance or induction heaters or else outer jackets or internal boresfor carrying a heat exchange fluid. In many cases a cooling jacket 12 ispreferred for purposes of the present invention (FIG. 4) although aheating jacket 13 may also be used (FIG. 5). i

All of the above mentioned embodiments have in common the feature thatheat transfer takes place through heat conduction within the melt regionaround each pin, because melt portions of differently converging partialmelt streams are constantly brought together with melt portions ofotherpartial melt streams. On the other hand, during the flowing aroundthe individual pin, there also takes place an additional heat transferthrough heat flow between the melt and the pin, since a relatively largeportion of the melt comes in contact with individual pins of the largenumber present in the metering zone.

The thermoplastic melt, which is more or less well homogenized, flows insubstantially laminar melt layers in the metering zone of the extruderis effectively and extremely efficiently mixed in one and/or anotherthreadless screw section provided in this metering zone by reason of thechosen design and] arrangement of the pins. Due to the large number oflong and short pins provided, the thermoplastic melt is subdivided intoindividual melt streams which flow in directions which are partlyaxially parallel, partly diagonal, partly orthogonal and partly radialwith reference to the screw axis. Simultaneously, individual meltstreams are conducted around the individual pins or additionallydirected over the ends of the short pins. In the flow of the meltthrough the remaining free interspaces or flow gaps between adjacentpins, differently spaced from pin to pin, the individual meltstreams aredifferently accel erated and after short flow paths around about thepins or over their ends, these individual streams are reunited withfurther individual streams and in part also superposed above or belowother individual streams.

According to one preferred embodiment as indicated above, all the pinsare approximately equal in diameter and arranged at a flow gap spacingof less than 1.5 pin diameters. Although more uniform or regularplacement of the pins in this manner reduces the randomness of flow,there is still an excellent mixing in the sense of achieving a highdegree of temperature equalization.

Where the pins are arranged in the manner of a herringbone pattern as inFIGS. 2 and 3, both the pin groups and the pin rows of the herringbonepattern and also the pin-free zones receive the constantly flowing meltin the various different directions indicated above, in which processthe hotter core melt is intensively mixed with outer peripheral orborder melt and vice versa. On the basis of the melt drag flow and themelt pressure flow, there is yielded a mean flow angle of the melt whichdoes not agree with the chosen angle, for example, of the axiallyangular pin row of the herringbone pattern arrangement. By reason of thepin section constructed free of thread flights on the screw core andbecause of the melt flow angle, all the melt components are brought intocontact with numerous pins over the entire flow path through the mixingsection of the invention. Through the constant but briefly repeatingsubdividing and uniting of a large number of individual melt streams asa result of the large number of long and short pins over a certainlongitudinal or axially elongated section, the thermoplastic meltbecomes increasingly homogeneous and much more equal with respect to itstemperature. The melt portions which flow around the pins or over theends of the pins are more or less regularly directed by the pins and themelt portions which flow through the pin-less areas flow, in comparisonto the aforementioned melt portions, in a more irregularly directedmanner. On the one hand, various divided melt streams unite frequentlywith others and, on the other hand, melt layers are superimposed ordifferent layers flow into one another and, in most instances, inlocally differing directions and with locally different flow speeds.

By reason of the large number of the pins provided in the mixing sectionof the invention, there is present a considerably greater surface ofthis screw section, as compared to that of any comparable known screws.In general the surface area of this screw is enlarged about 100 percentby the invention.

According to further features of the invention, the radial projectionsare preferably constructed in the form of cylindrical pins. For thepurpose of the most favorable flowing of the melt over the pins, theseare shown to be rounded on the end surface 14 away from the screw coreand facing the inner wall of the housing 2 (see FIG. 3). The pins canalso have differing diameters, so that through these measures there arearranged a number of pins of alternatingly different thickness. Theclear spacings or flow gaps between the pins are thereby more variable.The melt partial streams, because of the difference in height anddiameter of the pins which results in a changing of the flow speed, aswell as because of the resulting irregular spacings, take on a veryirregular course or flow path which leads to a very irregular dividingand reuniting of the individual melt streams.

The pins are preferably arranged shrunk-fit into the threadless coresection of the screw using conventional heat-resistant and corrosionresistant metals common in screw extruders. The removal of theconventional thread flight is accomplished, for example, by milling itoff over the presecribed length. In the event that a threadless sectionis provided at the screw end, there can also be provided a separatepin-carrying or radially studded cylindrical part whose diameter, withinlimits, is somewhat greater or smaller than the screw core diameter.This modified screw end or head can be provided as an extension whichcan be screwed onto the metering section of the extruder screw.

The radial extension of the long pins 5, as measured between pin apex orouter end l4.and the screw axis,

is about equal to or up to only 1 percent smaller than the screw radius;the radial extension of the shorter pins 6 with otherwise equal diameterand position is about 4 to 12 percent smaller than the screw radius.

Tests have been carried outwith the apparatus of the invention in which,on the basis of the new mixing section apparatus, the temperaturedifferential (A T) amounts to less than 1 C. in the discharged melt, andmore particularly, that temperature difference measured between the meltflow at the core of the screw and the melt flow at the outer peripheryalong the barrel housing. The temperature of the melt flowing from thenovel mixing section of the screw was thereby approximately equal atevery point of the flow crosssection. Furthermore, the desired melttemperature was controlled and maintained at a favorable value as wasdesired for its discharge into a die connected at the outlet end of thescrew extruder. This is especially important if the melt, after leavingthe extruder, is to pass into a distributor line system before beingextruded, e.g., as with spinning melts for films, filaments and thelike.

Comparative values were obtained from futher experiments using a screwwhich had pins located within a screw channel between two conventionalthread flights, and this comparison exhibited a A T in the range between5 and 8 C. over the same comparable flow cross-section. In all the testscarried out, the melts were approximately equally uniform with respectto the de-' gree of mixing and the degree of homogenization, but thiswas not the case with respect to the equalization of the temperature.The improved temperature evening or equalization within a A T of lessthan 1 C. was achieved only with the new mixing section, and thisrepresents a significant advance over prior devices where a A T of 5-8C. is common and a A T of less than 3 C. has not generally beenattainable.

The screw extruder of the invention with its special mixing section isbroadly useful with all synthetic thermoplastic polymers, especiallythose used as filmforming or fiber-forming polymer melts such aspolyamides (nylons), polyesters (polyethylene terephthalate) andpolyolefins (polyethylene and polypropylene) or closely relatedmodifications of such polymers. Together with the desired temperatureequalization, the invention is capable of uniformly mixing andhomogenizing such polymers at high throughputs even with the relativelylong (2D) mixing section with its numerous flow-deflecting pins.

The mixing and heat-equalizing processes which can be accomplished withthe aid of the new mixing element take place without excessive pressureor undesirable shearing influences on the melt. There are no undulynarrow gaps between the individual radial pins or between the pins andthe inside wall of the housing that have to be flowed through or aroundunder disadvantageous conditions.

Those partial melt streams which are directed diagonally in the mixingsection are essentially those that flow between the pins and around thepins. The orthogonally directed partial streams are essentially thosethat flow through the pin-free -zones while the radially directedstreams are substantially those that flow off over the short pins. Theaxially parallel as well as axially angularly directed melt streamsgenerally result in the uniting of coverging individual streams. Suchomnidirectional flow is believed to be at least partly responsible forthe excellent mixing and temperature equalization while avoiding localhot spots and/or melt stagnation.

Thus, by means of the cyclically repeating subdividing, uniting, layersuperimposing and layer mixing of the melt portions from variouslydirected melt streams and with the large surface area present in themixing section, including the surface of the thread-free and pin-freescrew core, and by reason of the interspaces present between the screwcore and the interior wall of the housing, there is brought about in theflow of the melt through the mixing section of the invention both anintensive intermixing and homogenization and also an extremely efficienttemperature equalization.

The radial projections of the mixing device of the invention, as notedabove, are preferably present as cy lindrical pins or closely equivalentelongated studs which are inserted by shrink fitting into easily drilledbores in the root or core of the screw. Such a device is not only simplein construction and highly efficient in use, but is also cheaplymanufactured from existing screws and can be easily installed inconventional barrel housings. Minor variations in structure and form areof course permissible within the scope of the invention.

The invention is hereby claimed as follows:

1. In a screw extruder for the continuous processing of a thermoplasticpolymer wherein a barrel housing surrounds a single rotatable screwhaving a metering zone toward the discharge end thereof, the improvementwhich comprises at least one section of the screw insaid metering zonewhich is free of thread flights over an axial length of at least twotimes the screw diameter and which carries a large number of pinsprojecting radially from the threadless screw core, the individual pinsbeing varied between at least two different free radial heights and theratio of pin diameter to said free height being from 0.25:1 to 2:1.

2. A screw extruder as claimed in claim 1 wherein said pins of differentheights are arranged in an irregular sequence.

3. A screw extruder as claimed in claim 1 wherein all of said pins haveapproximately the same diameter and are arranged next to one another toprovide a flow gap therebetween of not more than 1.5 pin diameters.

4. A screw extruder as claimed in claim 1 wherein pins of different freeradial heights are arranged in groups along the ,core surface such thatsaid threadless core section also contains zones which are free of pins.

5. A screw extruder as claimed in claim 4 wherein said pin groups arearranged in a herringbone pattern.

6. A screw extruder as claimed in claim 1 wherein said pins areessentially cylindrical in shape.

7. A screw extruder as claimed in claim 6 wherein the end surfaces ofthe pins facing the inner wall of the housing are rounded.

8. A screw extruder as claimed in claim 1 wherein all of said pins arecomposed of a highly polished metal which is heatand corrosion-resistantand which has a good heat-conducting capacity.

9. A screw extruder as claimed in claim 1 wherein the distance from thescrew axis to the apex of the longest pins is equal to the screw radiusor up to 1 percent less than the screw radius while the correspondingdistance of the shorter pins is within a range of about 4 to 12 percentless than the screw radius.

10. A screw extruder as claimed in claim 1 wherein the radial pins arecarried over a length of the screw extending into a screw channeladjacent that section of the screw which is free of thread flights.

2 UNITED STATES PATENT OFFECE CERTIFICATE 6F EORRECTION Patent No. 3,755 Dated Augu 7 r 975 Inventor(s) Friedhelm Hensen et a1 It is certifiedthat error appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

V First page, left-hand ,column, eleventh line. irisert L 30 j ForeignApplication Priority Data June 23, 197;) k Germany P 20 50 756.6 7

Column 2, line 4,- "form these cams" should read form Column 7, line 58,"presecribed" should need prescribed Signed and sealed this 27th day ofNovember 1973 Attest: H

EDWARQMHPLBTgHERQR. RENE r5. TBGTMEYBR v Attestlng Office-r ActingCommissioner of Paiehts

1. In a screw extruder for the continuous processing of a thermoplasticpolymer wherein a barrel housing surrounds a single rotatable screwhaving a metering zone toward the discharge end thereof, the improvementwhich comprises at least one section of the screw in said metering zonewhich is free of thread flights over an axial length of at least twotimes the screw diameter and which carries a large number of pinsprojecting radially from the threadless screw core, the individual pinsbeing varied between at least two different free radial heights and theratio of pin diameter to said free height being from 0.25:1 to 2:1.
 2. Ascrew extruder as claimed in claim 1 wherein said pins of differentheights are arranged in an irregular sequence.
 3. A screw extruder asclaimed in claim 1 wherein all of said pins have approximately the samediameter and are arranged next to one another to provide a flow gaptherebetween of not more than 1.5 pin diameters.
 4. A screw extruder asclaimed in claim 1 wherein pins of different free radial heights arearranged in groups along the core surface such that said threadless coresection also contains zones which are free of pins.
 5. A screw extruderas claimed in claim 4 wherein said pin groups are arranged in aherringbone pattern.
 6. A screw extruder as claimed in claim 1 whereinsaid pins are essentially cylindrical in shape.
 7. A screw extruder asclaimed in claim 6 wherein the end surfaces of the pins facing the innerwall of the housing are rounded.
 8. A screw extruder as claimed in claim1 wherein all of said pins are composed of a highly polished metal whichis heat- and corrosion-resistant and which has a good heat-conductingcapacity.
 9. A screw extruder as claimed in claim 1 wherein the distancefrom the screw axis to the apex of the longest pins is equal to thescrew radius or up to 1 percent less than the screw radius while thecorresponding distance of the shorter pins is within a range of about 4to 12 percent less than the screw radius.
 10. A screw extruder asclaimed in claim 1 wherein the radial pins are carried over a length ofthe screw extending into a screw channel adjacent that section of thescrew which is free of thread flights.